August 18, 2014

Balmer or Balmy?

left image from LRO QuickMap, right map from Hawke and colleagues (2005), and center image is a mashup of the two. Balmer is the flooded crater just below the center. The dark marking in the map is mare lava, and the hatched pattern is for cryptomaria (dark lava covered by lighter material); A, B, C etc are crater ejecta deposits overlying maria.

All impact basins formed in the first 600-700 million years of the Moon's existence. Presumably none formed later because by then projectiles large enough to cause one had collided with something or became trapped in a stable (asteroidal) orbit. The later basins such as Orientale, Imbrium and Nectaris are pretty well preserved, but earlier ones were modified so that their existence and dimensions are quite uncertain. The Balmer, or Balmer-Kapteyn Basin is one of these poorly known, but probable early basins. It was speculated to exist by Bill Hartmann and Chuck Wood in 1971, but was first convincingly delineated by Ted Maxwell and Connie Andre ten years later. The map used here comes from a study by R. Ray Hawke and colleagues in 2005. The evidence for the existence of the basin is that there is a flat, smooth plain in the space between the craters Balmer and Kapteyn, and that there are mountainous rim fragments on the NE and NW rims of Balmer, and on the SW rim of Kapteyn. Ray Hawke and friends detected 25 dark halo craters punching through the plain that show that in some places mare basalts underly the plain. Typically, but not always, mare lavas fill basin floors, so the existence of buried maria, along with the fragmentary basin rims, is suggestive that B-K is indeed an old basin. But it's in such bad shape that the outer rim diameter has been suggested to be 450 km in diameter, or 550 km. The inner ring is estimated at 225 km width. The B-K Basin has not yet been re-examined using the LRO data, but at least one dark-halo crater has appeared as an LROC Featured Site.